Kinetic studies of lead(II) uptake by the seagrass Zostera muelleri in water by radiotracing,atomic absorption spectrometry and electrochemical techniques

Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent.     

Upper limits on the total radiant energy of solar flares

We establish limits on the total radiant energy of solar flares during the period 1980 February – November, using the solar-constant monitor (ACRIM) on board the Solar Maximum Mission. Typical limits amount to 6 × 10²⁹ erg/s for a 32-second integration time, with 5σ statistical significance, for an impulsive emission; for a gradual component, about 4 × 10³² ergs total radiant energy. The limits lie about an order of magnitude higher than the total radiant energy estimated from the various known emission components, suggesting that no heretofore unknown dominant component of flare radiation exists.     

A Yohkoh search for “black-light flares”

Calculations which predict that a phenomenon analogous to stellar negative pre-flares could also exist on the Sun were published by Hénouxet al. (1990), and Aboudarhamet al., (1990), who showed that at the beginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under certain conditions. They named this event a black light flare (BLF). Such a BLF event should appear as diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with intense and impulsive hard X-ray bursts. The BLF location would be at (or in the vicinity of) the forthcoming bright patches. Their predicted contrast depends on the position of the flare on the solar disc and on the wavelength band of the observation.TheYohkoh satellite provided white-light data from the aspect camera of the SXT instrument (Tsunetaet al., 1991), at 431 nm and with a typical image interval of 10–12 s. We have studied nine white-light flares observed with this instrument, with X-ray class larger than M6. We have found a few interesting episodes, but no unambiguous example of the predicted BLF event. This study, although the best survey to date, was not ideal from the observational point of view. We therefore encourage further searches. Successful observations of this phenomenon on the Sun would greatly strengthen our knowledge of the lower solar atmosphere and its effects on solar luminosity variations.     

Gamma radiation and photospheric white-light flare continuum

Recent gamma-ray observations of solar flares have provided a better means for estimating the heating of the solar atmosphere by energetic protons. Such heating has been suggested as the explanation of the continuum emission of the white-light flare. We have analyzed the effects on the photosphere of high-energy particles capable of producing the intense gamma-ray emission observed in the 1978 July 11 flare. Using a simple energy-balance argument and taking into account hydrogen ionization, we have obtained the following conclusions:

1. Heating near τ₅₀₀₀ = 1 in the input HSRA model atmosphere is negligible, even for very high fluxes of energetic particles.
2. Energy deposition increases with height for the inferred proton spectra, and does not depend strongly upon the assumed angle of incidence. The computed energy inputs fall in the range 10–100 ergs (cm³ s)^?1 at the top of the photosphere.
3. H^? continuum dominates for column densities as small as 10²² cm^?3, but at greater heights hydrogen ionizes sufficiently for the higher continua to dominate the energy balance.
4. The total energy deposited in the ‘photospheric’ region of H^? dominance could be within a factor of 3 of the necessary energy deposition, by comparison with the white-light flare of 1972 August 7, but the emergent spectrum is quite red so that the intensity excess in the visible band is insufficient to explain the observations.

In summary, it remains energetically possible, within observational limits, that high-energy protons could cause sufficient heating of the upper photosphere to produce detectable excess continuum, but emission from the vicinity of τ = 1 is not significant.     

Solar flares,microflares, nanoflares,and coronal heating

Solar flare occurrence follows a power-law distribution against total flare energy W: dN/dW W ^– with an index 1.8 as determined by several studies. This implies (a) that microflares must have a different, steeper distribution if they are energetically significant, and (b) there must be a high-energy cutoff of the observed distribution. We identify the distinct soft distribution needed for coronal heating, if such a distribution exists, with Parker's nanoflares.This paper considers a microflare to be a member of the normal X-ray burst population, with comparable physical parameters except for a smaller total energy.     

Hydrologic and geomorphic processes in the Colville River delta, Alaska

The Colville basin drains the North Slope of Alaska and is one of several large Arctic river systems located within permafrost. The timing and style of fluvial processes in the earth's permafrost regions differ from those occurring in midlatitude settings. Moreover, in comparison to temperate-zone systems, rivers located entirely within permafrost perform most of their work during relatively short periods of time. This paper examines river ice hydrology and the resulting geomorphic processes that occur within the Colville delta, Alaska. Fluvial processes and landform development within the Colville delta occur after the flood-pulse is initiated by the breakup of river ice. During this 4-month period, the geomorphic processes are largely influenced by the movement of ice. The flood-pulse and accompanying river ice influences erosional and depositional processes and results in unique styles of sediment transport, deposition, and riverbank erosion.     

Comment on: 'The 3D shear experiment over the Natih field in Oman: the effects of fracture-filling fluids on shear propagation' by C.M. van der Kolk, W.S. Guest and J.H.H.M. Potters

We find errors in theory and application in the paper by van der Kolk et al. (Geophysical Prospecting 49 , 179–197 (2001)) that invalidate their arguments for the cause of the reduction in the velocity of the vertically propagating slower split shear-wave in the gas cap over the Natih field in Oman. We suggest that existing theories are adequate and can explain the anomaly.     

Time-domain computation of scattering from a heterogeneous layer

Adopting Born and ray approximations, time-domain synthetic seismograms for P-P and P-S scattering from a plane wave incident on a thin, laterally heterogeneous layer are presented in this paper. The time-domain P coda is a convolution between a structure function and the second-order derivative of the time function of the incident P wave. Examples of synthetic seismograms are given using a time function from a computed short-period seismogram for a point explosive source in a half-space. These show that it is impossible, with realistic values of the parameters involved, to generate significant codas when only single scattering is involved.